Re: [PATCH V9 7/9] vhost: introduce vDPA-based backend

[Jason Wang]

On Tue, Nov 2, 2021 at 11:56 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Tue, Nov 02, 2021 at 11:52:20AM +0800, Jason Wang wrote:
> > On Mon, Nov 1, 2021 at 10:11 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Mar 26, 2020 at 10:01:23PM +0800, Jason Wang wrote:
> > > > From: Tiwei Bie <tiwei.bie@xxxxxxxxx>
> > > >
> > > > This patch introduces a vDPA-based vhost backend. This backend is
> > > > built on top of the same interface defined in virtio-vDPA and provides
> > > > a generic vhost interface for userspace to accelerate the virtio
> > > > devices in guest.
> > > >
> > > > This backend is implemented as a vDPA device driver on top of the same
> > > > ops used in virtio-vDPA. It will create char device entry named
> > > > vhost-vdpa-$index for userspace to use. Userspace can use vhost ioctls
> > > > on top of this char device to setup the backend.
> > > >
> > > > Vhost ioctls are extended to make it type agnostic and behave like a
> > > > virtio device, this help to eliminate type specific API like what
> > > > vhost_net/scsi/vsock did:
> > > >
> > > > - VHOST_VDPA_GET_DEVICE_ID: get the virtio device ID which is defined
> > > >   by virtio specification to differ from different type of devices
> > > > - VHOST_VDPA_GET_VRING_NUM: get the maximum size of virtqueue
> > > >   supported by the vDPA device
> > > > - VHSOT_VDPA_SET/GET_STATUS: set and get virtio status of vDPA device
> > > > - VHOST_VDPA_SET/GET_CONFIG: access virtio config space
> > > > - VHOST_VDPA_SET_VRING_ENABLE: enable a specific virtqueue
> > > >
> > > > For memory mapping, IOTLB API is mandated for vhost-vDPA which means
> > > > userspace drivers are required to use
> > > > VHOST_IOTLB_UPDATE/VHOST_IOTLB_INVALIDATE to add or remove mapping for
> > > > a specific userspace memory region.
> > > >
> > > > The vhost-vDPA API is designed to be type agnostic, but it allows net
> > > > device only in current stage. Due to the lacking of control virtqueue
> > > > support, some features were filter out by vhost-vdpa.
> > > >
> > > > We will enable more features and devices in the near future.
> > >
> > > [..]
> > >
> > > > +static int vhost_vdpa_alloc_domain(struct vhost_vdpa *v)
> > > > +{
> > > > +     struct vdpa_device *vdpa = v->vdpa;
> > > > +     const struct vdpa_config_ops *ops = vdpa->config;
> > > > +     struct device *dma_dev = vdpa_get_dma_dev(vdpa);
> > > > +     struct bus_type *bus;
> > > > +     int ret;
> > > > +
> > > > +     /* Device want to do DMA by itself */
> > > > +     if (ops->set_map || ops->dma_map)
> > > > +             return 0;
> > > > +
> > > > +     bus = dma_dev->bus;
> > > > +     if (!bus)
> > > > +             return -EFAULT;
> > > > +
> > > > +     if (!iommu_capable(bus, IOMMU_CAP_CACHE_COHERENCY))
> > > > +             return -ENOTSUPP;
> > > > +
> > > > +     v->domain = iommu_domain_alloc(bus);
> > > > +     if (!v->domain)
> > > > +             return -EIO;
> > > > +
> > > > +     ret = iommu_attach_device(v->domain, dma_dev);
> > > > +     if (ret)
> > > > +             goto err_attach;
> > > >
> > >
> > > I've been looking at the security of iommu_attach_device() users, and
> > > I wonder if this is safe?
> > >
> > > The security question is if userspace is able to control the DMA
> > > address the devices uses? Eg if any of the cpu to device ring's are in
> > > userspace memory?
> > >
> > > For instance if userspace can tell the device to send a packet from an
> > > arbitrary user controlled address.
> >
> > The map is validated via pin_user_pages() which guarantees that the
> > address is not arbitrary and must belong to userspace?
>
> That controls what gets put into the IOMMU, it doesn't restrict what
> DMA the device itself can issue.
>
> Upon investigating more it seems the answer is that
> iommu_attach_device() requires devices to be in singleton groups, so
> there is no leakage from rouge DMA

Yes, I think so.

Thanks

>
> Jason
>